There are no available data documenting the dialysis clearance of iodide in humans. This work quantitates the hemodialysis clearance of iodide (as 131I) over time and examines certain factors which influence that clearance. In a single-patient study, three dialysis periods were studied over the 24 to 96 hours following administration of 129 mCi 131I given as Na131I. Hemodialysis clearance of 131I was calculated both from the arterio-venous difference of 131I across the artificial kidney and the appearance of 131I in the expended dialysate. Calculations were based on 131I activity in whole blood, whole plasma, and the supernate of plasma treated with 10% trichloroacetic acid (TCA). The hemodialysis clearance of 131I was highest when calculated from the activity of the plasma supernate, 171.3 +/- 6.0 ml/min for the period 24-29 h. post dosing, and fell in a linear fashion with time. The clearance calculated from whole blood activity was always intermediate to that from whole plasma (lowest) or the plasma supernate (highest). The percentage of plasma 131I activity precipitated by TCA rose over the study period and displayed a strong negative correlation to the hemodialysis clearance of 131I. The hemodialysis clearance of iodide, as 131I, is on the order expected for a free ion and is 4-5 times higher than the endogenous renal clearance of the ion. However, the calculated clearance varies with respect to total time of dialysis and the fraction of blood in which the 131I activity is measured.(ABSTRACT TRUNCATED AT 250 WORDS)